Oscillator circuits comprising transistors as active elements have been used for a long time. With the arrival of integrated circuits, ready made amplifiers of that kind have been used as active elements in oscillators. For high frequency use, only some types of electronic circuits are suitable for instance directly coupled amplifiers. One kind of these amplifiers comprises ECL-circuits (ECL is here Emitter Coupled Logic), for which the output signal is taken from the emitter of an output transistor. The corresponding output terminal further must be connected by means of an extra outside resistor, a pull down resistor, to ground in order to have the voltage on the emitter to be defined. This extra resistor thus is not included in the integrated circuit.
ECL-circuits in addition have a high input impedance, approximately like the base of a transistor, and a low output impedance like the emitter of a transistor, typically 6-9 Ohms. An ECL-amplifier typically has a gain of about 5.
As amplifying elements in oscillators, circuits comprising ECL line receivers, i.e. ECL differential amplifiers having a differential output with a non-inverting and an inverting output signal, have been proposed. These are usually utilized for converting signal like sinus waves from other oscillators or amplifiers to the electric levels used by ECL-circuits or generally for amplification of other signals to ECL-levels.
Oscillator circuits using ECL line receivers are for instance illustrated in "MECL System Design Handbook", Motorola Semiconductor Products Inc., 4th edition 1988, pps. 224-228.
For generating high frequencies, high frequency crystals using harmonic overtones must be used. In some respects, they are not suitable and therefore fundamental tone crystals are generally used. They thus have a lower frequency, which requires a frequency multiplication of the lower frequency.
A frequency multiplication circuit using a differential amplifier of the integrated circuit type, having non-inverting and inverting input terminals and output terminals is disclosed in the German Offenlegungschrift DE A1 39 30 126. To both of the input terminals, the same DC bias is applied and the frequency to be multiplied is fed to one of the input terminals. The other input terminal is connected to ground for signal frequencies through a decoupling capacitor. The output terminals are directly connected to each other, i.e. short-circuited with each other, which also can be expressed as "wired-OR". In this way, an output signal of a sine wave type is obtained. The way described of driving the amplifier, however, will give on the output terminals thereof, only a signal having a frequency corresponding to the double frequency of the input signal.